fix: guard against decorators in interfaces (#3266)

Author: tserg

tests/parser/syntax/test_interfaces.py

@@ -5,6 +5,7 @@
     ArgumentException,
     InvalidReference,
     InvalidType,
+    StructureException,
     SyntaxException,
     TypeMismatch,
     UnknownAttribute,
@@ -76,6 +77,14 @@ def test(a: address):
     """,
         SyntaxException,
     ),
+    (
+        """
+interface A:
+    @external
+    def foo(): nonpayable
+    """,
+        StructureException,
+    ),
 ]
 
 

vyper/semantics/types/user.py

@@ -468,6 +468,10 @@ def _get_class_functions(base_node: vy_ast.InterfaceDef) -> Dict[str, ContractFu
             raise NamespaceCollision(
                 f"Interface contains multiple functions named '{node.name}'", node
             )
+        if len(node.decorator_list) > 0:
+            raise StructureException(
+                "Function definition in interface cannot be decorated", node.decorator_list[0]
+            )
         functions[node.name] = ContractFunctionT.from_FunctionDef(node, is_interface=True)
 
     return functions